Stress:
When any object of certain material is subjected to tensile or compressive or shear force, there may be a chance that its dimension will be affected.
If the dimension of a material is changed due to application of forces we can say that the material is under stress or experiencing the state of stress.

When the force applied is perpendicular to the cross section of the object then the stress is known as normal stress. (The force can be tensile or compressive)

Shear Stress:

When the force applied is parallel to the material cross section then the stress is known as shear stress.

Stress Strain Graph for Mild Steel (Ductile Material)

If a relatively small force is applied to ductile material steel and it starts to deform (that means that the steel is in stress and we can measure the strain) then we will a curve which is initially a straight line. After plotting stress vs strain we will get the relation.

In this stress strain graph of mild steel we will find a straight line , that is up to point A from the origin. From that experimental graph we can conclude that stress is proportional to strain. Up to point A is the limit of proportionality. Within the elastic limit the deformation of the steel will be temporary. After the withdrawal of the force the steel bar will return to its original shape. If the force is increased then steel bar will be deformed elastically up to point B. This is the elastic limit (Point B). Beyond that if the force is increased then the plastic deformation will start and we will have upper yield point C and lower yield point D. For further increase in the force material will experience fracture or breaking stress (Max. Ultimate Stress). From this graph we get Young's modulus of elasticity for steel which is 210 GPa. This graph also associates modulus of resilience as well as modulus of toughness.

Extrusion Process Principles

Extrusion process is very common in the
plastic industry. It is used for high volume plastic production. In extrusion
process the color pigments along with other performance enhancing additives are
combined with resin which is then pushed through the rotating screws. The heat
and pressure generated within the barrels of screw is dispersed and melts the
plastic elements. Thus a homogeneous mixture is produced. A cool die is usually
placed at the end of the mixture. After passing through the die the mixture is
ready to experience the finishing operation which may include pelletizing,
calendaring or other processes.

The arrangements shows a motor which
runs the gear box. The speed is used by this gear reducer box. Which then moves
the screw. The metal ingredients is poured through the hopper. The feeding of
metal is done in the feeding zone. Then with the increased pressure and
temperature of the screw melts the elements in the melting zone. The next part
is the melt pumping zone which increases the pressure of the liquid. This
pressurized liquid is then pushed through the filter. Then the molten metal
flows through the die and cooled. The thermocouple is set at different points
to measure the temperature.

The figure of the whole process is shown below. Click on it to have a better view .

Advantages of Plastic Extrusion Process

One of the main advantages of the
extrusion process is it can produce pipes of any length. It is continuous. High
production volumes with very low cost per pound. The melting is very efficient.
The mixing of the ingredients is very good. Different types of raw materials
can be used. It uses thermoplastics which can be reused.

Disadvantages of Plastic Extrusion
Process

Highly complicated parts are very not
suitable for production in extrusion process. It can only produce the parts
which have uniform cross sections.

Plastics can be classifies into two broad categories -
Thermosets and Thermoplasctics. A brief description of these two classes will be discussed here.

Thermosets are the kind of plastics which are not recyclable. They undergo a kind of chemical change at the time of heating. As a result they can be shaped once because of this permanent chemical change. This process is known as heat hardening. They are also known as thermosetting plastics. Examples : Epoxy Resin, Melamine formaldehyde , polyester resin , Urea formaldehyde.

Thermoplastics are the recyclable types. While heating they undergo a temporary chemical change. After the heat is removed they retain their original shape. The process is known as heat softening. Examples : Polyamide (Nylon), Polymethyl methacrylate, polypropelene, polystyrene, Low density Polythene (LDPE), High density Polythene (HDPE).

Cope and drag in Sand Mold :

Cope and Drag are the two parts of the casting flask. Cope is the upper part and drag is the lower part. Even if the casting process is flaskless , the same terms are used for the upper ans lower parts. Generally the flask is made of wood or metal. It contains molding sand. When metal is poured into the mold cavity the flask supports the mold.

Gating system:

If the molten metal is poured directly from the ladle , it will erode the bottom of the mould cavity. So molten metal is poured from the ladle to the cavity through a gating system. The gating system in casting creates a series of channels through which molten metal reaches the cavity. Gating system has -

Pouring Basin : It receives the molten metal from the liquid metal container.

Sprue : Pouring cup is attached to the sprue. It is vertical in shape . On the other part of the sprue there is part called runner.

Sprue Base : Its the base of the sprue

Runner : It is the horizontal part of the gating system. It connects the spues with the gates.

The next part is the choke.

Then comes the skim bob

Gates and ingates : It controls the movement of the metal from the runners into the cavity.

Riser

Riser:

The risers are also known as feed heads. When the metal solidifies it starts to shrink. And then risers comes into play. These feeder supply metals to the cavity when shrinking of the metal starts.

Core:

Cores are required to create the castings with holes. It can be made of refractory materials. Most often core sand is used to make it. Metal cores are also available but less frequently used.

Chaplets:

Chaplets are the supports for the cores. These are needed particularly when the cores are very big. Usually metal pieces are used to support the core. Without chaplets the core can be displaced and the casting can be spoiled. These chaplets are set-up between the core and mold surface. Caution should be taken while placing chaplets. Clean, oil and moisture free pieces should be used as chaplets.

Chills:

These are huge metal pieces used to reduce the effect of shrinkage. These increases the thermal conductivity and heat capacity. It helps in speeding up the cooling process. So thick metal parts are cooled quickly. They can be used along with the risers.

Investment or lost-wax castings:
The Lost-wax method, sometimes, also called simply as precision-investment casting has been used for many years by jewelers and dentists. Since world-war-II, the method has been adapted to the production of small and precise, industrial fasting. Basically, the method involves the use of expendable (heat disposable) pattern unrounded with a shell of refractory material to form the casting mould. Example: Piston making.
Cast parts are formed by putting the molten metal in the cavity. The wax pattern is melted and it is not reusable because of that the method is named "lost - wax method".

The steps involved in this Lost Wax Method method are explained below:

Wax injection

Assembly

Shell building

Dewax

Conventional casting

Knockout

Cut-off

Finished castings

(a) Wax patterns are usually made by injection molding

(b) Multiple Patterns are assembled in central wax sprue .

(c) The assembly is immersed in a liquid ceramic slurry. After that this structure is dipped into a bed of very fine sand. More than one layer of sand may be necessary.

(d) The ceramic becomes dry. The wax is then melted. To melt all the wax ceramic is fired.

(e) Simple gravity pouring is used for filling the mould. After solidification all the parts of the ceramic structure become casting. Hollow castings can be made by pouring out the extra metal before solidification.

(f) The metal cools to room temperature and after that the ceramic structure is broken off by water blasting or by applying vibration.

(g) The parts are separated by the high speed friction saw. Minor finishing is given to the parts.

Die casting process is very different from sand casting as the mould used here is permanent. The mould is not expandable and need not to be broken after the cooling and solidification. Here the mould is called the die.

When molten metal is injected into the permanent metallic mould by the means of the external pressure it is known as die casting or pressure die casting. In this process the liquid metal is forced into the permanent die.

Applications of pressure die casting

Cast parts obtained from pressure die casting are used in automobiles , electrical equipment, motors, machine parts, telecommunication equipment, building materials, auto ancillary and many other sectors. Home appliances and children's toys also use pressure die cast parts .

In sand casting which is also known as sand molded casting, an object is produced by sand mold. The process involves pouring of the molten metal in to the mold cavity. The molten metal is then cooled to the room temperature. The metal is solidified. After cooling, the metal object is separated from the mold. Sand casting process has its advantages and disadvantages. So care should be taken while making delicate products.

Core Making

What is Core ?

It is a structure made of refractory material . It is prepared before pouring the molten metal on to the cavity. There is another term known as core print which is the projection on a pattern used to make spaces in the mould. It is also known as core seat.

Criteria for selecting a furnace for casting

Capacity needed to hold the molten metal

Melting rate

Quality of the melt

Temperature needed

Method of pouring

Operation in Cupola Furnace

Cupola is the most popular furnace for casting non ferrous and ferrous materials . It is known as a shaft furnace. The shell of cupola furnace is made of steel of 8-10 mm thickness. The shell is supported by columns or legs. The inside of the furnace is lined with refractory material to save the furnace from over heating.

And lastly comes the casting defects. These defects can occur for different reasons like air trapping in the sand mold, low quality sands, improper ramming , defecting sand mold etc. Go through this link for detailed knowledge of casting defects.

Pattern allowances:

There are some allowances which are responsible for the difference in the dimensions of the casting and the pattern. These allowances are considered when a pattern is designed for casting. In this article we will discuss those allowances -

1.Shrinkage allowance:

After solidification of the metal from further cooling (room temp.) dimensions of the patterns increases. So pattern size is bigger than that of the finished cast products. This is known as shrinkage allowance.

It depends on:

a)Dimensions of casting

b)Design and intricacy of
casting

c)Resistance of mol to
shrinkage

d)Molding materials used

e)Method of molding used

f)Pouring temp of the molten
metal

2.Draft or taper allowance:

Pattern draft is the taper placed on
the pattern surfaces that are parallel to the direction in which the pattern is
withdrawn from the mould (that is perpendicular to the parting plane), to allow
removal of the pattern without damaging the mould cavity.

It depends on:

a)the
method of molding

b)the sand
mixture used

c)the design
(shape and length of the vertical side of the pattern)

d)economic
restrictions imposed on the casting

e)intricacy
of the pattern

3.Distortion allowance:

This allowance is taken into consideration when casting products of irregular shapes. When these are cooled they are distorted due to metal shrinkage.

4.Finishing or machining allowance:

Machining allowance or finish allowance indicates how much
larger the rough casting should be over the finished casting to allow sufficient
material to insure that machining will "clean up" the surfaces.

This machining allowance is added to all surfaces that are to
be machined. Machining allowance is larger for hand molding as compared to machine
molding.

It depends on:

a)Machining operation

b)Characteristics of metal

c)Methods of castings

d)Size, shapes and volumes of castings

e)Degree of finish required in castings

f)configuration
of the
casting

5.Shaking or rapping allowance:

To take the pattern out of the mould cavity it is slightly
rapped to detach it from the mould cavity. So the cavity is increased a little.

Brazing is a gas welding process in which you join metals by using heat that surpasses the 800 degree Fahrenheit mark and a nonferrous (iron-free) filler rod with a melting temperature below that of the base materials. The most important point about brazing is that you can use it to join dissimilar metals — cast iron to steel, brass to steel, or copper to steel, just to name a few examples.Keeping a few brazing rules in mindA successful brazing job requires that you stick to a few rules, as follows:✓ The surfaces of the metals must be free of contaminants. Use steel wool to clean off all the metals, and use flux (a material that dissolves or removes oxides and other contaminants from the surface during brazing) for additional cleaning when the surfaces are heated.✓ The joint to be brazed must have a tight fit. You use two basic joints for brazing: the butt joint (two pieces of material lying together on the same plane) and the lap joint (two pieces of material overlapping each other, usually in a parallel plane). If the joint has sizeable gap, brazing just doesn’t work. But if the joint is too tight, the melted filler rod can’t penetrate the entire joint, and you get a weak, ineffective weld.✓ The base metals you’re brazing must remain stationary during theheating and cooling process. If the base metals move while you’re welding or before everything has cooled off after welding, the joint’s integrity will likely be compromised

Giving brazing a tryIf you’re trying brazing for the first time, I recommend going with an oxyacetylene torch in the flat position and using the forehand method I discuss in “Making the weld” earlier in the chapter. You can start by creating a brazed corncob, which is a piece created when you join two different metals,following the steps in this section.

Brazing principles

1. Acquiring a section of carbon steel pipe 1 inch in diameter and five inches long, as well as four 1⁄8-inch fluxed brazing filler rods that are 36 inches long.

2. Clean the pipe thoroughly with steel wool to remove any contaminants on the surface.

3. Lay the pipe between two fire bricks, leaving a 3⁄4-inch space between the bricks.Fire bricks are special bricks that can withstand extremely high temperatures; find them at your welding supply or hardware store.4. Set up and light the oxyacetylene torch, using the steps I describe in “Working through the Basics of Welding with Gas” earlier in this chapter, and adjust the torch so you have a neutral flame.5. Preheat the pipe to burn off any grease or varnish that may be left onthe surface.6. If you’re right-handed, start on the right end of the pipe and melt off a small portion of the filler rod onto the end of the pipe. If you’re left-handed, start on the left end of the pipe. The molten puddle should be very fluid. Be sure that when you use more of the filler rod, you let only the molten puddle (not the flame from the torch) melt the rod. If you notice white smoke coming from your molten puddle, that means you’re burning the zinc out of your filler rod, which will result in a poorweld. Avoid that problem by welding at a lower temperature or moving more quickly with the molten puddle. When you’re practicing the brazing process, you can quench the metal between passes with the torch. Use pliers to pick up the section of pipe you’re practicing on and place it in a tank of water to cool it very quickly. Quenching isn’t good for the integrity of the brazed weld, though, so don’t quench unless you’re just practicing.

7. After you make your first pass with the torch, start a second pass by pointing the tip of the flame at the edge of the previous pass; when part of the first braze starts to melt, add some of your filler rod and proceed. This pass laps over the first braze bead 1⁄3 to 1⁄5.

8. When you’ve completed your second pass, quench the welded metal (or allow it to cool) and go ahead and start on a third pass. When you’re finished with the third pass, you should have a finishedproduct that resembles the brazed corncob in Figure 13-3. For some brazing projects, you may need to make even more passes. (For example, projects involving thick pieces of metal definitely require more than three passes.)